1. Field of the Invention
The present invention relates to positive-electrode active material particles for an all-solid battery, and a production method for the positive-electrode active material particles.
2. Description of Related Art
In recent years, secondary (rechargeable) batteries have become essentially important as power sources for devices, such as personal computers, video cameras and cellular phones, or power sources for vehicles and for storage of electric power.
Among secondary batteries, lithium ion secondary batteries, in particular, have higher energy density and are operable at a higher voltage than other secondary batteries. Thus, lithium ion secondary batteries are used in information-related devices and communication devices as secondary batteries that can be easily reduced in size and weight. In recent years, high-output, high-capacity lithium ion secondary batteries for low-emission vehicles, such as electrical vehicles and hybrid vehicles, are being developed.
A lithium ion secondary battery or lithium secondary battery includes positive electrode layers, negative electrode layers, and an electrolyte. The electrolyte contains a lithium salt and is interposed between the positive electrode layer and the negative electrode layer. The electrolyte is composed of a non-aqueous liquid or solid. When a non-aqueous liquid electrolyte is used as the electrolyte, the electrolytic solution penetrates into the positive electrode layer. Thus, an interface is readily formed between the electrolyte and a positive-electrode active material that constitutes the positive electrode layer. As a consequence, the performance of the battery improves. However, because electrolytic solutions that are typically used are flammable, a safety device is provided to prevent temperature rise in the event of short-circuit. Alternatively, a system to ensure safety, such as one that prevents short-circuit, is installed. On the other hand, all-solid batteries, which use a solid electrolyte in place of a liquid electrolyte to create an all-solid structure, do not include a flammable organic solvent therein. Thus, all-solid batteries, which are conceivable to allow the safety device to be simplified and excel in production cost and productivity, are being developed.
In all-solid batteries, in which a solid electrolyte layer is interposed between the positive electrode layer and the negative electrode layer, the positive-electrode active material and electrolyte are solid. Thus, the electrolyte does not tend to penetrate into the positive-electrode active material and the interface between the positive-electrode active material and the electrolyte tends to decrease. Therefore, in all-solid batteries, a composite material that contains a mixed powder of a positive-electrode active material powder and a solid electrolyte powder is used in the positive electrode to increase the area of the interface.
Use of sulfide-based solid electrolyte as the solid electrolyte for, in particular, all-solid batteries is being examined. The sulfide-based solid electrolyte has excellent lithium ion conductivity. However, when the sulfide-based solid electrolyte is used, an interface resistance to the lithium ions that migrate through an interface between the active material and the sulfide-based solid electrolyte tends to increase. This is conceivable to be because the active material reacts with the sulfide-based solid electrolyte to form high-resistance portions in the surface of active material. The increase in interface resistance leads to a decrease in the performance of the all-solid battery. To address the increase in interface resistance, several techniques to prevent the increase in interface resistance have been disclosed. For example, a technique to reduce the interface resistance by coating the surface of the active material with, for example, lithium niobate has been disclosed (International Publication No. 2007/004590 (A1)).
In addition, the positive-electrode active material of an all-solid battery is composed of a metal oxide. Therefore, the positive electrode of the all-solid battery has poor electron conductivity. Thus, coating positive-electrode active material particles with a conductive aid is being examined. For instance, carbon and the like are used as the conductive aid. To coat active material particles with carbon, various methods, such as mechano-chemistry and pulsed laser deposition (PLD), may be used. Among these methods, mechano-chemistry, such as mechano-fusion, is often used. The mechano-fusion is high productivity method for forming a coat using mechanical energy, such as compression, friction or impact. (Japanese Patent Application Publication No. 2008-270175 (JP 2008-270175 A).
As described above, it is being examined to coat each of the active material particles with layers that inhibits a reaction between the active material and the sulfide-based solid electrolyte (which is hereinafter referred to as “reaction-inhibiting layer”) in order to reduce the interface resistance between the active material and the sulfide-based solid electrolyte. On the other hand, it is also being examined to coat each of the active material particles with conductive aid to improve the electron conductivity therebetween.
It is, however, proved that when active material particles that have been respectively coated with reaction-inhibiting layers are coated with carbon by mechano-chemistry, such as mechano-fusion, mechanical stress which is generated by mechano-chemistry makes each of the reaction-inhibiting layers easy to peel off the active material particle. FIG. 1 is a cross-sectional schematic view of a particle of an active material according to a related art, which is obtained by coating an active material core which has been coated with a reaction-inhibiting layer with carbon by mechano-fusion.
As shown in FIG. 1, an active material particle 10 includes an active material core 11 and a reaction-inhibiting layer 12 that covers the active material core 11. When the active material particles 10 are coated with carbon by mechano-fusion, carbon layers 23 are respectively formed on the surfaces of the active material cores 21 of active material particles 20. However, the mechanical stress that is generated during the mechano-fusion makes some parts of the reaction-inhibiting layer 22 easy to peel off. Thus, regions with a reaction-inhibiting layer 22 and regions with no reaction-inhibiting layer 22 may be present on the surface of the active material core 21. There is a possibility that high-resistance portions are formed in the regions with no reaction-inhibiting layer 22 and decrease the performance of the battery.